Corynebacterium Diphtheriae

Introduction

Corynebacteria are Gram-positive, aerobic, nonmotile, rod-shaped bacteria classified as Actinobacteria. Corynebacteria are related phylogenetically to mycobacteria and actinomycetes. They do not form spores or branch as do the actinomycetes, but they have the characteristic of forming irregular, club-shaped or V-shaped arrangements in normal growth. They undergo snapping movements just after cell division, which brings them into characteristic forms resembling Chinese letters or palisades (Bonnet, pp 242-9).

The genus Corynebacterium consists of a diverse group of bacteria including animal and plant pathogens, as well as saprophytes. Some Corynebacteria are part of the normal flora of humans, finding a suitable niche in virtually every anatomic site, especially the skin and nares. The best known and most widely studied species is Corynebacterium diphtheriae, the causal agent of the disease diphtheria.

Figure 1: Stained Corynebacterium cells. The "barred" appearance is due to the presence of polyphosphate inclusions called metachromatic granules. Note also the characteristic "Chinese-letter" arrangement of cells.

Discussion

Diphtheria is an upper respiratory tract illness characterized by sore throat, low fever, and an adherent membrane (called a pseudomembrane on the tonsils, pharynx, and/or nasal cavity. Diphtheria toxin produced by C. diphtheriae, can cause myocarditis, polyneuritis, and other systemic toxic effects. A milder form of diphtheria can be restricted to the skin (Groman, pp 48-56).

Diphtheria is a contagious disease spread by direct physical contact or breathing aerosolized secretions of infected individuals. Once quite common, diphtheria has largely been eradicated in developed nations through wide-spread use of the DPT vaccine. For example, in the U.S., between 1980 and 2004 there were 57 reported cases of diphtheria. However, it remains somewhat of a problem worldwide (3,978 reported cases to WHO in 2006) in the face of efforts to achieve global vaccination coverage.

Diphtheria is a serious disease, with fatality rates between 5% and 10%. In children under 5 years and adults over 40 years, the fatality rate may be as much as 20%. Outbreaks, although very rare, still occur worldwide, even in developed nations. Following the breakup of the former Soviet Union in the late 1980s, vaccination rates in the constituent countries fell so low that there was a surge in diphtheria cases(Chen et al, pp 1393-7). In 1991 there were 2,000 cases of diphtheria in the USSR. By 1998, according to Red Cross estimates, there were as many as 200,000 cases in the Commonwealth of Independent States, with 5,000 deaths.

Pathophysiology

Humans comprise the only reservoir of infection. Carriers are usually asymptomatic. C diphtheriae spreads via respiratory droplets, contact with nasopharyngeal secretions, or wound exudates in cases of cutaneous disease. Rarely one may contract diphtheria via fomites. Overcrowding, poor health, and substandard living conditions facilitate the spread of the disease(Bonnet, pp 242-9).

C diphtheriae is not a particularly invasive organism. It occupies the superficial layers of the respiratory tract and the skin causing local tissue inflammatory reaction followed by tissue necrosis. The occurrence of systemic disease depends on the elaboration of a potent exotoxin by tox+ strains. The diphtheria toxin is a 62,000-dalton polypeptide composed of 2 joined segments (A and ...